The present exemplary embodiments relate to a process for forming molded or extruded composites and the products formed thereby, particularly automobile weather strips. They finds particular application in conjunction with vehicle weather strip composites comprised of an elastomeric polymer and a show layer including a thermoplastic vulcanizate and a crosslinked thermoplastic polyolefin, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
It is common in the motor vehicle industry to fashion decorative abrasion resistant sections for various parts of an automobile by extruding or molding such sections from certain thermosetting polymeric materials. Examples of typical abrasion resistant sections manufactured by such a process include colored weather strips. These weather strips are mounted on an automobile door surface and along the perimeter of automobile doors to provide a seal between the door and the automobile body as well as to protect both the door and exterior objects when they come in contact with each other. Weather strips are typically molded or extruded and attached to a vehicle by an adhesive tape or by mechanical means such as by crimping or the use of fasteners.
Various thermoset elastomeric rubber materials, such as ethylene propylene diene terpolymer (EPDM), styrene-butadiene copolymer (SBR) and chloroprene rubbers have been commonly used to form these weather strips. These materials are favored by manufacturers because they are relatively inexpensive compared to thermoplastics and generally exhibit both the desired flexibility necessary for providing an effective seal and acceptable weatherability properties. However, these elastomers typically lack the low-friction, abrasion resistance, and weatherability that is necessary at the point of contact with the exterior for extended life of the weather strips. In addition, it is difficult to impart desirable surface color and gloss to such materials.
Manufacturers have therefore attempted a variety of approaches to improve the wear resistance, aesthetics and other properties of elastomeric sealing sections. One strategy for weather strips has been to apply a second layer of low friction polymer to selected surfaces of the elastomeric weather strip, particularly along an area that is exposed to the exterior. Incorporated within the second layer can be various pigments or dyes such that the surface of the weather strip matches the color of the automobile.
Depending on the composition of the main body of the weather strip, this second layer is often formed from polyvinyl chloride (PVC) or an uncured non-polar thermoplastic elastomer, such as polypropylene or polyethylene. These second layers are usually applied directly to the weather strip surface by lamination or as a solvent-based spray, or after an application of a primer or adhesive layer to the elastomer. However, these methods are not completely satisfactory. In addition to longer processing time and added material cost, it is difficult to obtain a satisfactory bond between the elastomer and the surface coating. Sprayed on coatings are prone to cracking while an adhered layer is susceptible to peeling.
Another method that manufacturers have used to adhere the second layer to the extruded weather strip is to cohesively bond a layer of wear resistant thermoplastic to the weather strip. Several techniques have been developed to accomplish this. According to one method, the elastomer rubber and the second layer are co-extruded. The resulting composite is then passed through an oven in which the elastomer rubber is cured and the interface between the second layer and the rubber is heated to such a degree that the second layer partially melts, causing it to adhesively bond with the rubber. Alternately, the rubber is extruded first and passes through an oven in which it is at least partially cured. A molten thermoplastic is then extruded onto the vulcanized rubber. The residual heat of the rubber as it emerges from the oven promotes interdiffusion of the two layers at the interface between the two, forming a bond between the two materials.
Due in part to the uncrosslinked nature of the thermoplastic, however, it is difficult to control exactly the degree of melting that the second layer undergoes in this technique. If the second layer melts too much, the abrasion resistance it affords may be compromised and its aesthetic appeal diminished. Thus, there is a need for a new vehicle weather strip composite that overcomes the deficiencies and limitations of the prior art.